include: usb: audio: specify exact endiannes of descriptors

USB spec says that multiple byte fields are stored in
little-endian order (see chapter 8.1 of USB2.0 spec and
chapter 7.1 of USB3.0 spec), thus mark such fields as LE
for UAC1 and UAC2 headers

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>